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## Copyright (C) 1995, 1996, 1997 Kurt Hornik
##
## This program is free software; you can redistribute it and/or modify
## it under the terms of the GNU General Public License as published by
## the Free Software Foundation; either version 2, or (at your option)
## any later version.
##
## This program is distributed in the hope that it will be useful, but
## WITHOUT ANY WARRANTY; without even the implied warranty of
## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
## General Public License for more details.
##
## You should have received a copy of the GNU General Public License
## along with this file. If not, write to the Free Software Foundation,
## 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
## usage: irr (p [, i])
##
## Computes the internal rate of return of a series of payments p from
## an initial investment i, i.e., the solution of npv (r, p) = i. If the
## second argument is omitted, i = 0 is used.
##
## See also: npv; pv, rate.
## Author: KH <Kurt.Hornik@ci.tuwien.ac.at>
## Description: Internal rate of return of an investment
function r = irr (p, i)
if (nargin == 1)
i = 0;
elseif !(nargin == 2)
usage ("irr (p [, i])");
endif
tmp = output_precision;
output_precision = 15;
if !(is_vector (p))
error ("irr: p must be a vector");
else
p_string = type p;
endif
if !is_scalar (i)
error ("irr: i must be a scalar");
endif
string = ["function delta = f (r) ", ...
"delta = npv (r, %s) - %g; end"];
eval (sprintf (string, p_string, i));
r = fsolve ("f", 0.01);
output_precision = tmp;
endfunction
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